HONOLULU — Humpback whales were spotted in the waters off Hawaii for the first time this season, leading officials to warn ships and recreational boaters to steer clear of the endangered visitors.

Researchers saw the first whale about a week ago off the small island of Niihau, near Kauai, the National Oceanic and Atmospheric Administration said in a news release Tuesday. A second whale was seen days later near Kauai’s Pacific Missile Range Facility.

Humpbacks normally arrive in the tropics in November and stay through May. There has been a general trend toward earlier arrivals in the past two decades, with the average in the past 10 years happening on Oct. 1, officials said. About 10,000 humpbacks winter in Hawaii each year.

Officials said boats should watch for and avoid the newly arriving whales. Ship strikes and entanglements can kill the animals and injure people on board.

While researchers have little data on the number of fatal whale encounters with people, they know that boat strikes are common.